Rechargeable lithium ion batteries have a negative electrode containing elemental lithium, which is often intercalated in some carbonaceous substance, a positive electrode bearing an oxide which is capable of incorporating lithium ions in its structure and an electrolyte containing mobile lithium ions, located between the negative and positive electrodes. The positive electrode may also contain a lithium compound having dissociable lithium ions, a binder mixed with the oxide, and fine carbon added to the mixture. The oxide in the positive electrode is usually a transition metal oxide. The electrolyte is commonly a solid organic polymer or a non-aqueous liquid, which has a lithium salt dissolved in it or contains dissociable lithium ions in some other form. The electrolyte may also be a microporous solid polymer which has been impregnated with an organic liquid containing a dissociable lithium salt. The electrolyte which is non-conductive of electrons, provides ionic passage for lithium ions only. Lithium ions move from the elemental lithium containing negative electrode or anode to the transition metal oxide containing positive electrode or cathode, on discharge of the battery. Lithium ions are moved from the cathode or positive electrode through the electrolyte to the negative electrode in the recharging step. The above lithium ion battery may be button shaped or be constructed of thin laminates forming a conventional thin film or planar battery. The external faces of the electrodes are in contact with current collectors, which are usually in the form of metal foil or metal sheet. Lithium batteries need to be protected from atmospheric corrosion and are usually enclosed in some form of air-tight container or polymer film.
One of the difficulties in manufacturing a lithium ion battery which is capable of prolonged service life, is to promote and maintain satisfactory electrical contact between the current collector and the face of the electrode adjacent to the current collector while also ensuring corrosion protection. There are conventional ways for exerting pressure on the contact surfaces or sealing the individual cells in a tough polymer wrapping under pressure, however, the exerted pressure may weaken in use, in particular in the case of flat batteries made of laminates, and/or the seal may be broken. Contact between the interactive surfaces, in particular between the current collector and the face of the positive electrode in contact with it, may diminish due to delamination caused by lack of adhesion or other factors, such as for example, released gases that had been absorbed by the contact surfaces. At any rate, conventional methods of applying and maintaining pressure on the electrode-current collector contact surfaces may add several costly additional steps to the battery manufacturing process, which may not even be effective in prolonged use.
There are known methods of providing a layer of carbon fibres or fine carbon particles, or an electronically conductive inorganic or organic polymer layer between the metallic current collector and the appropriate surface of the elemental lithium bearing negative electrode for providing the required electric contact. In another approach in providing contact between the electrode and the current collector, Koksbang et al. in U.S. Pat. No. 5,368,959, issued on Nov. 29, 1994, describe an electrically conductive organic polymer layer on the external face of the transition metal compound containing positive electrode of a lithium battery, for taking the role of the current collector. Manufacturing an electronically conductive polymer layer which is also tough, may notably increase production costs, and may not prevent delamination between the contact surfaces of a lithium battery.
In a different field of alkaline batteries utilizing zinc anode and manganese dioxide cathode, T. L. Dunham, in U.S. Pat. No. 5,278,006 issued on Jan. 11, 1994, teaches a nickel plated steel clip acting as current collector which has a layer of platinum, rhodium or palladium coated on the clip facing the manganese dioxide cathode to improve electrical contact. As is well known, plating precious metals on contact surfaces is an expensive process, moreover it would not prevent delamination of the battery, nor would a precious metal coating assist in maintaining contact between the interactive surfaces of a lithium battery pack.
There is a need for an inexpensive method for maintaining good electrical contact in a lithium battery between a conventional metallic foil or sheet metal current collector and a transition metal oxide bearing positive electrode.